Batrachotoxin (BTX, 1), homobatrachotoxin, and batrachotoxinin A comprise a small family of complex steroidal alkaloids originally isolated in sparing amounts from the skin of Colombian poison dart frogs of the genus Phyllobates. See Marki and Witkop, Experientia, 1963, 19, 329; Daly et al., J. Am Chem. Soc., 1965, 87, 124. β-Hydroxybatrachotoxin and 4β-hydroxyhomobatrachotoxin were also isolated as minor components, see: Tokuyama and Daly, Tetrahedron, 1983, 39, 41. Batrachotoxin acts as a selective agonist of voltage-gated sodium channels (NaVs) and is among the most potent non-peptidic toxins known (LD50 in mice=2 μg/kg). Tokuyama et al., J. Am Chem. Soc., 1969, 91, 3931; Albuquerque et al., Science, 1971, 172, 995. The binding of BTX to NaVs results in a remarkably complex array of responses, including hyperpolarization of threshold activation, elimination of inactivation gating, and reduction of single channel conductance. Wang and Wang, Cell. Sign., 2003, 15, 151; Quandt and Narahashi, Proc. Nat. Acad. Sci. USA, 1982, 79, 6732. While there exist other small molecule modulators of NaVs, arguably none show activity that is as multifaceted as BTX. Severely limited quantities of BTX, however, frustrate any efforts to evaluate structure-function relationships and to utilize BTX or select analogues to interrogate mechanisms of ion selectivity and channel gating. The sacrifice of over 10,000 Colombian poison dart frogs, which are now endangered, resulted in the isolation of 180 mg of BTX and 100 mg of homoBTX (Garraffo and Spande, Heterocycles, 2009, 79, 195), and frogs raised in captivity do not produce the toxin (Daly et al., Science, 1980, 208, 1383).

Batrachotoxin offers numerous intriguing challenges to the chemist interested in de novo synthesis. Tokuyama et al., J. Am Chem. Soc., 1968, 90, 1917; Karle and Karle, Acta Crystallogr., 1969, B25, 428; Gillardi, Acta Crystallogr., 1970, B26, 440. The E-ring homomorpholine is unique among secondary metabolite structures, and the 9α-hydroxy 3β-hemiketal, C16-C17 unsaturation, and 20 cc pyrrole ester do not appear in other steroidal natural products. To date, only a single de novo route to (±)-batrachotoxinin A has been reported (Kurosu et al., J. Am Chem. Soc., 1998, 120, 6627), although Imhof and co-workers completed a partial synthesis of an analog of (−)-batrachotoxinin A in which the stereochemistry of the C20 alcohol was inverted (S instead of R) (Imhof et al., Helv. Chim. Acta, 1973, 56, 139; Imhof et al., Helv. Chim. Acta, 1972, 55, 1151). The impressiveness of the BTX synthesis notwithstanding, the length and linear nature of the synthesis does not provide a viable means to BTX analogues. Other reports describe preparations of A/B/C ring system variants, but the variants that have been tested show modest to little effect as NaV modulators. Schumaker and Keana, J. Chem. Soc. Chem. Comm., 1972, 622; Keana and Schumaker, J. Org. Chem., 1976, 41, 3840; Magnus et al., J. Chem Soc. Chem. Comm., 1985, 1185; Hudson et al., Tet. Lett., 1993, 34, 7295; Trudeau and Deslongchamps, J. Org. Chem., 2004, 69, 832; Lacrouts et al., Synlett., 2005, 18, 2767; Schow et al., Bioorg. Med. Chem. Lett., 1997, 7, 181. The preparation of the C/D/E skeleton has not been reported, in spite of the fact that mouse lethality and electrophysiology data indicate that variations to the C, D, or E rings dramatically alter BTX activity. Khodorov et al., Cell. Molec. Neurobio., 1992, 12, 59; Warnick et al., J. Pharmacol. Exper. Ther., 1975, 193, 232. Without intending to be bound by theory, homology modelling and protein mutagenesis data suggest that BTX binds to the inner pore region of NaV through primary contacts with the C/D/E ring unit. Tikhonov and Zhorov, FEBS Lett., 2005, 579, 4207; Du et al., J. Biol. Chem., 2011, 286, 13151; Wang and Wang, Biophys. J., 1999, 76, 3141; Wang and Wang, Proc. Nat. Acad. Sci. USA, 1998, 95, 2653; Wang et al., Pflugers Arch., 2007, 454, 277; Du et al., Biochem. J., 2009, 419, 377; Wang et al., Mol. Pharmacol., 2001, 59, 1100; Wang et al., Channels, 2007, 1, 179.
Compounds with structures unrelated to that of BTX have been reported to compete with the binding of BTX to NaV and to display isoform-specific NaV blocking effects. PCT International Publication No. WO2010/027641.
The strong therapeutic potential of batrachotoxin in the treatment of pain and other diseases of the nervous system make it an attractive candidate for structural modification and analysis. There is thus a need for improved compound analogues of batrachotoxin, pharmaceutical compositions, methods of use, and methods of preparation.
Effective delivery of an active compound to a desired target is also of major importance in all types of pharmacotherapy. Transdermal delivery of drugs represents an attractive alternative to oral or parenteral delivery, and, over the last few decades, has made an important contribution to medical practice. See Prausnitz and Langer, Nature Biotechnology, 2008, 26, 1261. Trans-dermal delivery has many advantages in comparison to other delivery routes. See, e.g., Prausnitz and Langer, Nature Biotechnology, 2008, 26, 1261; Jepps et al., Advanced Drug Delivery Reviews, 2013, 65, 152. Oral analgesics, for example, are commonly prescribed for the treatment of acute and chronic pain, but often produce adverse systemic effects such as potentially fatal respiratory depression, nausea, and addiction. Argoff, Mayo Clinic Proceedings, 2013, 88, 195. Clinically effective drug concentrations can be introduced at a peripherally located site of injury or inflammation by topical administration without resulting in high systemic concentrations that may increase the likelihood of adverse effects. Argoff, Mayo Clinic Proceedings, 2013, 88, 195; McCleane, Medical Clinics of North America, 2007, 91, 125. Nevertheless, the main challenge in topical applications and transdermal drug delivery is the penetration through the skin barrier. Determining the permeability of a given compound through the human skin is quite difficult, however, owing to the highly complex nature of the skin structures and the various proposed mechanisms that constitute the delivery pathway. Jepps et al., Advanced Drug Delivery Reviews, 2013, 65, 152.